1. Field of the Invention
The present invention concerns electrically conducting complexes, methods of producing such complexes and their use in producing highly conducting plastic materials. More particularly, the present invention concerns such a complex which comprises two components, the complete dissolution of which must be avoided.
In the complex according to the present invention, there is a highly conductive component (component (A)) and a second component (component (B)), which is capable of dissolving component (A). In the present invention, component (A) and component (B) are combined so that limited dissolution takes place at the interfaces between the parts, whereby the advantages of each component are obtained in the complex. The invention also concerns a method of producing the complex and the use of such complexes together with a polymer matrix in highly conductive plastic materials.
2. Description of the Related Art
Currently, electrically conducting polymers are attracting great interest worldwide. Such polymers offer the possibility of replacing metallic conductors and semiconducting materials in a plurality of applications including batteries, sensors, switches, photocells, circuit boards, heating elements, antistatic protection (ESD) and electromagnetic interference protection (EMI). Conducting polymers have the advantages over metals of light weight, corrosion resistance, and lower production and processing costs.
Conducting polymers can be roughly categorized into two different groups: filled conducting polymers, which contain a conductive filler, e.g. carbon black or lampblack, carbon fiber, metal powder, etc., added to a thermosetting or thermoplastic resin; and intrinsically conducting polymers and complexes, which are based on polymers made conductive by an oxidation, reduction or protonation (doping) process.
The electrical conductivity of filled conducting polymers is dependent on the mutual contacts formed between the conductive filler particles. Typically, approximately 10 to 50% by weight of well-dispersed filler material is required to achieve composites of high conductance. However, problems are associated with such conducting composite materials: the mechanical and other properties of such composites are decisively degraded as the filler content increases and the polymer content decreases; their conductivity becomes difficult to control particularly in the semiconductor range; and stable and homogeneous dispersing of the filler into the matrix polymer becomes difficult.
Intrinsically conducting polymers can be produced from organic polymers having long chains formed by conjugated double bonds and heteroatoms. The polymers may be made into conductive complexes by modifying the .pi.- and .pi.-p electron systems of the double bonds and heteroatoms in the polymers by adding into the polymer certain doping agents. Thus, the backbone chain of the polymer can be modified to contain electron holes and/or excess electrons that provide pathways for the electric current along the conjugated chain.
The benefits of intrinsically conducting polymers and complexes include easy modification of their conductivity as a function of the dopant concentration, also described as the doping level, which is particularly accentuated in conjunction with low conductivities. By contrast, attaining low conductivities with filled conducting polymers is difficult. Examples of kinds of polymers known in the art as intrinsically conducting polymers include polyacetylene, poly-p-phenylene, polypyrrole, polythiophene and its derivatives and polyaniline and its derivatives.
Plastics are processed into desired articles, such as workpieces, fibers, films, etc., by two major types of processes: melt processing and solution processing. Melt processing is suitable for multiple applications, while solution processing can be used principally only in the manufacture of fibers and films, and is not generally suitable for making shaped articles. However, the processing and doping of most intrinsically conducting polymers result in problems with the handling, stability, homogeneity and other aspects of these materials when processed into conducting plastics.
An intrinsically conducting polymer that is particularly technically and commercially promising is polyaniline and its derivatives. Polyaniline is an aniline polymer or its derivative which is based on aniline monomers or their derivatives, in which the nitrogen atom is bonded to the para-carbon in the benzene ring of the next unit. Polyaniline can occur in several forms, such as leucoemeraldine, protoemeraldine, emeraldine, nigraniline and toluoprotoemeraldine. For conducting polymer applications, the emeraldine form, having the formula ##STR1## wherein x is approximately 0.5, is usually used.
Doping of polyaniline is performed in accordance with methods known in the art by conventionally using protonic acids including among others HCl, H.sub.2 SO.sub.4, HNO.sub.3, HClO.sub.4, HBF.sub.4, HPF.sub.6, HF, acids of phosphorus, sulfonic acids, picric acid, n-nitrobenzoic acid, dichloroacetic acid and polymeric acids. Doping is advantageously performed with a sulfonic acid and most advantageously with dodecylbenzene sulfonic acid (DBSA). Protonation attacks the nonprotonated nitrogen atoms of the aniline units shown in the formula above, the proportion of such nonprotonated nitrogen atoms being approximately 50% of all N-atoms of the emeraldine base form of polyaniline. Herein reference is made to U.S. Pat. Nos. 3,963,498, 4,025,463 and 4,983,322, which are representative examples of the publications in the art. Numerous references to the forming of conductive complexes by doping of polyaniline with protonic acids may also be found in other literature in the art.
Conductive complexes of polyanilines doped with a protonic acid have been found extremely useful when blended with an excess amount of the protonic acid such as the above-mentioned sulfonic acid or its derivative, whereby the blend contains a sufficient amount of acid for both the doping and plasticization of the blend. In fact, using excess amounts of the protonic acid in this manner makes the doped polyaniline complex suitable for melt-processing, as the protonic acid serves the above two functions in the blended compound. However, such use of excess protonic acid gives doped polyaniline an acidic pH value, which acidity may decidedly hamper the use of the conducting polymer in most applications.
U.S. Pat. No. 5,340,499, which is incorporated by reference, discloses a method of plasticizing a conducting polymer complex containing polyaniline doped with a protonic acid, advantageously a sulfonic acid and most advantageously dodecylbenzene sulfonic acid. In the method according to the cited publication, the polymer blend containing doped polyaniline is treated with a metallic compound. According to the preferred embodiment of the method, the compound suited for plasticizing the doped polyaniline is prepared by reacting a metallic compound, most advantageously zinc oxide, with any acid capable of forming, with the metallic compound, a compound that acts as a plasticizer for the doped polyaniline. Such an acid is advantageously the same acid as that used for doping, namely, dodecylbenzene sulfonic acid (DBSA). The reaction mixture is heated and the plasticizing metallic compound thus formed is dried, cooled, and milled prior to being blended with the doped polyaniline. To transform the doped polyaniline into a processable form, the solidification method based on heat treatment disclosed in U.S. Pat. No. 5,346,649, which is incorporated herein by reference, is used.
Accordingly, the above-described method provides, most advantageously using a ZnO/DBSA compound, a less acidic, electrically conducting polyaniline plastic, which is further blended with a suitable matrix polymer such as polyethylene, to achieve the required mechanical properties. Thus, the zinc compound acts in this kind of blend as a plasticity and/or compatibility improving agent between the conducting polymer and the matrix polymer.